Recently, significant progress has been made in conceptually describing the dynamic aspects of coarse particle entrainment, which has been explored experimentally for open channel flows. The aim of this study is to extend the application of energy criterion to the low mobility aeolian transport of solids (including both natural sediment and anthropogenic debris such as plastics), ranging from incomplete (rocking) to full (rolling) entrainments. This is achieved by linking particle movements to energetic flow events, which are defined as flow structures with the ability to work on particles, setting them into motion. It is hypothesized that such events should impart sufficient energy to the particles, above a certain threshold value. The concept's validity is demonstrated experimentally, using a wind tunnel and laser distance sensor to capture the dynamics of an individual target particle, exposed on a rough bed surface. Measurements are acquired at a high spatiotemporal resolution, and synchronously with the instantaneous air velocity at an appropriate distance upwind of the target particle, using a hot film anemometer. This enables the association of flow events with rocking and rolling entrainments. Furthermore, it is shown that rocking and rolling may have distinct energy thresholds. Estimates of the energy transfer efficiency, normalized by the drag coefficient, range over an order of magnitude (from about 0.001 to 0.0048 for rocking, up to about 0.01, for incipient rolling). The proposed event-based theoretical framework is a novel approach to characterizing the energy imparted from the wind to the soil surface and could have potential implications for modelling intermittent creep transport of coarse particles and related aeolian bedforms.

Studies of microplastics (MPs) in remote, trans-boundary and alpine rivers are currently lacking. To understand the sinks and transport mechanisms of MPs, this study investigated the distributions and sources of MPs in the surface waters and sediments of five tributaries of the Koshi River (KR), a typical alpine river in the Himalayas between China and Nepal. Mean abundances of MPs in water and sediment were 202 +/- 100 items/m3 and 58 +/- 27 items/kg, dry weight, respectively. The upstream tributary, Pum Qu in China, had the smallest abundance of MPs, while the middle tributary, Sun Koshi in Nepal, had the greatest abundance. Compared to international values in rivers, contamination of the KR with MPs was low to moderate. Fibers represented 98% of all MP particles observed, which consisted of polyethylene, polyethyleneterephthalate, polyamide, polypropylene, and polystyrene. Blue and black MPs were prevalent, and small MPs (<1 mm) accounted for approximately 60% of all MPs. Atmospheric transmission and deposition were considered to be the principal sources of MPs in the upstream tributary. The results imply that point sources associated with mostly untreated sewage effluents and solid wastes from households, major settlements, towns, and cities were most important sources of MPs in the KR. Non-point sources from agricultural runoff and atmospheric transport and deposition in the middle stream tributaries also contribute a part of microplastics, while the least amount was from fishing in the downstream tributary. Urbanization, agriculture, traffic, and tourism contributed to pollution in the KR by MPs. Equations to predict abundances of MPs based on river altitudes revealed that different trends were affected by both natural and human factors within the KR basin. This study presents new insights into the magnitude of MP pollution of a remote alpine river and provides valuable data for developing MP monitoring and mitigation strategies in similar environments worldwide.